Chloride-induced stress corrosion cracking

Duplex stainless steels are mostly composed of alternate austenite and ferrite grains. Their structure improves resistance to chloride-induced stress corrosion cracking. In certain reducing acids, such as acetic and formic, preferential attack of the ferrite is a serious problem. 

Nickel-chromium alloys can be used in place of austenitic stainless steels where additional corrosion resistance is required. These alloys are still austenitic but are highly resistant to chloride-induced stress corrosion cracking when their nickel content exceeds 40 per cent. 

Titanium is immune to chloride induced stress-corrosion cracking but more expensive than type 300 series stainless steels. 

Type 304 stainless steel, one of the most commonly used alloys in a petroleum refinery, is very vulnerable to chloride-induced stress-corrosion cracking. Since there is no way fo stop stress-corrosion cracking once it starts, it is best not to expose Type 304 stainless or other similar alloys to a chloride environment. 

U-tube heat-exchanger bundles will have residual bending stresses. The bend may be vulnerable to chloride-induced stress-corrosion cracking unless the tubes are annealed to relieve the stresses. Type 304 stainless steel is also subject to stress-corrosion cracking when exposed to caustic solutions. 

In the austenitic CrNi and CrNiMo steels, chloride-induced stress corrosion cracking may occur at temperatures above about 323 K (50 °C) [167]. With higher contents of molybdenum, and in particular nickel, the resistance to stress corrosion cracking is increased. At normal environmental conditions, stress corrosion cracking is therefore not to be expected on these steels in seawater [168]. 

Because of their lack of susceptibility to chloride-induced stress corrosion cracking, nickel-copper alloys are used in seawater desalination plants as pipes for evaporators or heat exchangers. An evaporator made of alloy 400 (NiCu 30 Fe, 2.4360) exhibited a corrosion rate of > 0.01 mm/a (> 0.4 mpy) after an exposure time of 225 days to a CaCl2 concentration of up to 35 % and a temperature of 433 K (160 C). The corrosion rate was 0.07 mm/a (2.76 mpy) for a NaCl solution saturated with water vapor and air at 366 K (93 °C) [73]. 

Many types of alloy can suffer stress-corrosion cracking (SCC) if subject to external stress or residual stress when in contact with corrosive media. Typically SCC is characterized by a low-ductility or brittle fracture. For example standard austenitic stainless steels, e.g. types 1.4301 or 1.4401, are susceptible to chloride-induced stress-corrosion cracking in a chloride-containing environment at a temperature above 50 C. Depending on the specific environment cracks propagate trans- or intergranularly chloride-induced SCC is characterized by trans-granular cracks (Fig. 1-12). 

Because alloy B-2 is nickel rich (approximately 70%), it is resistant to chloride-induced stress corrosion cracking. Because of its high molybdenum content, it is highly resistant to pitting attack in most acid chloride environments. 

Alloy G is intended for use in the as-welded condition, even under the circumstances of mulfipass welding. The Columbian addition provides better resistance in highly oxidizing environments than does titanium additions. Because of fhe nickel base, the alloy is resistant to chloride-induced stress corrosion cracking. The 2% copper addition improves the corrosion resistance of the alloy in reducing acids, such as sulfuric and phosphoric. Alloy G will resist combinations of sulfuric acid and halides. 

Outstanding resistance to pitting, crevice corrosion, and intercrystalline corrosion Almost complete freedom from chloride-induced stress corrosion cracking High resistance to oxidation at elevated temperatures up to 1050°C Good resistance to acids, such as nitric, phosphoric, sulfliric, and hydrochloric, as well as to alkalis makes possible the construction of thin structural parts of high heat transfer... 

Figure 2.2 Chloride-induced stress corrosion cracking. (Reproduced with permission from Wesfarmers...

Figure 3.7 A sleeve showing signs of chloride-induced stress corrosion cracking. The crack is at right angles to the retaining screw point which is the point of maximum stress.

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